The present invention relates to hemming machines used for reentrantly folding edge portions of metal panels to provide a smooth finished edge surface.
Hemming machines are utilized to reentrantly fold an edge portion of a primary panel against itself or against a secondary panel thereby to reinforce the primary panel edge and present a smooth finished edge surface. Initially, the edge of the primary panel is folded in the stamping process to an angle of approximately 90.degree. relative to the surface thereof. The folded edge of the panel accommodates superposition of a secondary panel in juxtaposed relation thereto. Thereafter, the upstanding edge flange on the primary panel is reentrantly folded or "hemmed" to finish the edge and to secure the secondary panel, if used.
In order to condition the upstanding flange on a panel for hemming, it is necessary to reduce the angle of the upstanding flange from 90.degree. relative to the panel to an angle short of its final folded position, for example 45.degree., but sufficient to insure proper folding of the flange when impacted by the hemming die. This is known in the art as "prehemming".
In commonly owned U. S. Pat. No. 5,005,398, incorporated by reference herein, a hemming machine is described which combines both prehemming and final hemming in a single machine and in a single cycle of operation. The machine is disclosed as utilizing a plurality of like gates which are operable pneumatically and are capable of relatively simple electronic control. The pneumatic cylinders are mounted on individual gates to provide for easy access. The gates can be operated individually or can be synchronized to hem all edges of a panel simultaneously. A unique mechanical toggle action linkage in combination with cam controlled movement of the hemming dies minimizes tolerance deviations in both the prehemming and the final hemming phases of the hemming cycle.
While the hemming machine taught in U. S. Pat. No. 5,005,398 operates effectively, the retracting path of the prehemming die carrier after completion of the prehem operation limits the hemming machine applicability to relatively flat panels. More particularly, the downward retraction of the prehemming die carrier does not allow the prehemming die to clear the outer bottom edge of a curved or arcuate panel after the panel fold has been prehemmed to 45.degree..